Coupled Mode Modeling
To Interpret Hybrid Modes and
Fano Resonances in Plasmonic Systems
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Abstract
By generalizing the concept of extinction
cross-section to complex
valued extinction cross-section we analyze the coupling between plasmon
modes in metallic dimers or quadrumers. Identifying the phase information
in the field scattered by subsets of the whole plasmonic system allows
to infer the formation of subradiant or super-radiant hybrid modes.
We also propose a phenomenological modeling based on the use of coupled
mode equations to deduce from rigorous calculations a quantitative
estimate of mutual coupling coefficients when only two modes interfere.
These coefficients determine the spectral position of hybrid modes.
This approach is applied to two interacting silver spheres; the parameters
of the energetic diagram are calculated as a function of the gap between
spheres. In the case of two identical spheres illuminated with a linearly
polarized light parallel or perpendicular to the dimer, only one hybrid
mode is excited. The phenomenological modeling is then applied to
a four-particle system, where the interaction between the initial
dipolar modes gives rise to Fano resonances. In a weak coupling regime
of the system, the asymmetric line profile in the extinction spectra
of the system emerges from the superposition of a broad super-radiant
mode and a sharp subradiant mode. A strong coupling regime is characterized
by a broadened subradiant mode and a larger Fano resonance. The sharpness
of the Fano resonance in the weak coupling regime makes this structure
well suited for sensing applications